1. Field of the Invention
The present invention relates to an optical image transmitting system, an optical image transmitting apparatus, an optical image receiving apparatus, and an optical image transmitting method, which are used for transmitting/receiving an image (referred to as “optical image information”) represented by spatial light intensity distribution. In particular, the invention relates to an optical image transmitting system, an optical image transmitting apparatus, an optical image receiving apparatus, and an optical image transmitting method, which are used for transmitting optical image information through free space or an optical waveguide like an image fiber.
2. Description of the Related Art
Nowadays, the Internet and other such information transmitting techniques are utilized everywhere in our daily life. Among those, an information transmitting technique employing an optical transmission medium such as an optical fiber or image fiber is expected to become further widespread because of its advantages such as a high transmission speed and a large transmission capacity. In particular, attentions are focused on future development of a transmission technique (also called “coherent optical transmission”) based on light interference on account of its superiority in reception sensitivity etc.
The coherent optical transmission technique is based on the following optical heterodyne technique. First, signal light is emitted from the transmission side. The signal light is an optical signal including predetermined information, for example, image information. Meanwhile, reference light having a frequency (wavelength) slightly different from that of the signal light is produced on the reception side, and superimposed on the signal light by way of a beam splitter etc. to produce interference light. The interference light has a frequency corresponding to a frequency difference between the signal light and the reference light (called “beat frequency”, for example). The interference light is detected by a photo detector like a photodiode, and analyzed to thereby derive the information in the signal light.
Provided that Ir and Is represent an intensity of the reference light and that of the signal light, respectively, and fif and Δθ represent frequency difference and phase difference between the reference light wave and the signal light wave, respectively, a heterodyne signal as expressed by the following expression is outputted from the photo detector (for example, see Yoshizawa and Seta “Optical Heterodyne Technology (revised edition)”, New Technology Communications (2003), p. 2).
Expression (1)i(t)∝Ir+Is+2√{square root over (IrIs)} cos(2πfift+Δθ)  (1)
The third term of the right side of the expression (1) indicates an AC electrical signal and the frequency fif thereof is equivalent to the beat frequency caused from the interference between the reference light and the signal light. The frequency fif of an AC component of the heterodyne signal is the beat frequency. The first and second terms of the right side of the expression (1) indicate DC components of the heterodyne signal and correspond to a signal intensity of background light of interference light.
JP2002-009705 A (claims) discloses an example of conventional optical transmission techniques. The document discloses a high frequency signal transmission method including the steps of: combining an optical signal, a first local oscillation light, and a second local oscillation light having a frequency different from the first local oscillation light by a predetermined amount; selecting predetermined two of a plurality of frequency components obtained in the combining step; and combining the two selected frequency components, whereby a reception sensitivity to an optical signal is improved.
Considering the usability for a user, a transmitting system capable of transmitting information in a larger volume at a time will gain a large market share in the future. However, with such conventional optical transmission techniques, it is difficult to realize a transmitting system capable of transmitting an optical signal including plural types of information and, in addition, selectively extracting target information from the plural kinds of information on a reception side. In particular, a method of transmitting plural types of information as a single optical signal to extract and obtain target image information has been still out of the realm of possibility.